This invention relates to synthesizing software models of semiconductor process flows that are most likely able to fabricate desired semiconductor devices. More particularly, this invention relates to synthesizing such semiconductor process flow models using inverse modeling techniques.
The production of semiconductor devices involves a fabrication process. A fabrication process involves many variables, such as, for example, dopant concentrations and distributions in a semiconductor, environmental conditions (e.g., humidity, temperature, and cleanliness), physical parameters (e.g., film thicknesses, wafer sizes, device sizes, and numbers of layers), and electrical parameters (e.g., AC and DC characteristics, threshold voltages, transconductances, and capacitances). Needless to say, manually selecting particular combinations of variables or particular values for such variables when attempting to determine a particular process flow for fabricating a desired device or device characteristic is extremely complex with little assurance of success.
Thus, prior to fabricating a desired semiconductor device or desired characteristic, a potential process flow is typically modeled in software and then simulated. A process flow embodies each step of a fabrication or manufacturing process. Process flow modeling is often done because actual fabrication runs are expensive and time consuming, thus rendering a trial-and-error approach impractical. Modeling helps predict which potential process flows are most likely able to produce a device having desired characteristics. Thus, by modeling potential process flows, unnecessary and costly fabrication runs can be avoided.
Process flows are typically modeled using a physical model and a given set of modeling parameters. Physical models typically embody the physical, electrical, and other tangible properties of a particular device that has already been fabricated. This approach, however, has several problems. One problem is that the parameters provided to the process model may be inaccurate or insufficient. This can occur when a user does not have data that sufficiently characterizes the device to be fabricated. The process modeling tool may not then be able to accurately model a process flow.
Another problem is that the physical models themselves are often inaccurate. Models may be plagued with inaccuracies because technological advances typically progress faster than the actual understanding of the technology. This may make it difficult to construct models that accurately simulate process flows that can be used to produce desired devices.
The effects of these problems may be mitigated, if not rendered negligible, by using an inverse modeling technique. An inverse modeling technique uses xe2x80x9creverse engineeringxe2x80x9d to develop (or synthesize) a process that can fabricate a desired device. This technique has been used in the fields of geophysics, electromagnetism, and biotechnology. However, inverse modeling in the field of semiconductor fabrication has been at best limited.
In view of the foregoing, it would be desirable to synthesize semiconductor process flow models using an inverse modeling technique.
It is an object of this invention to synthesize semiconductor process flow models using an inverse modeling technique.
In accordance with this invention, semiconductor process flow models for fabricating desired semiconductor devices are synthesized using an inverse modeling technique. Inverse modeling, as opposed to conventional modeling, first models a desired device having desired characteristics (e.g., a transistor having properties for low voltage turn-ON and rapid frequency response). Inputs to this device modeling stage include measured data from previously fabricated devices, data from previously modeled process flows, desired characteristics from circuit designers, physical model parameters, and various numerical techniques and calibration methodologies that manipulate data.
After the desired device is modeled, various parameters are extracted from the modeled device. Extracted parameters may include, for example, structural characteristics (e.g., topography and film thickness), dopant levels, electrical characteristics (e.g., effective channel length and activation energy). These parameters are provided as inputs to a process modeling stage. This modeling stage also receives inputs that include best-guess modeled process flow information from previous process flow model simulations. The process modeling stage also iterates through an optimization loop, which optimizes modeled process flows in accordance with various constraints derived from the extracted parameters (e.g., design criteria), known fabrication processes (e.g., reliability and yield criteria), and known software tools (e.g., various tool limitations).
One or more process flows (i.e., process flows with different sets of parameters or ranges of parameters) that should be capable of fabricating the desired semiconductor device may be modeled by the process modeling stage. If more than one process flow is modeled, the constraints used to optimize the process flows are then modified iteratively until a unique process flow model is synthesized at the process modeling stage.
The resulting unique process flow model can then be simulated to fine tune or trade off selection of certain process parameters or variables. Upon satisfactory simulation results, an experimental fabrication run based on the modeled process flow can then be made. Measured data from the experimentally-fabricated devices can be fed back to the device and process modeling stages to further refine or modify the modeled process flow before a next experimental fabrication run is made.